1. Technical Field of the Invention
The present invention relates, in its more general aspect, to an emitter switching configuration, i.e., to a transistor configuration which comprises a cascode connection of a bipolar transistor having high breakdown voltage and a low voltage Power MOSFET. In particular, this invention relates to a driving circuit of an emitter switching configuration to control the saturation level in applications providing highly variable collector currents.
2. Description of Related Art
The emitter-switching configuration, consisting in the cascode connection of a power bipolar transistor, having high breakdown voltage, and a low voltage Power MOSFET, as shown in FIG. 1, has been known from the 1980's.
Nowadays, such configuration is made particularly interesting by the presence on the market of bipolar transistors having a squared RBSOA (in emitter-switching configuration) at a current being near the peak one and at a voltage equal to the BVCES, as well as of a Power MOSFET having a very low inner resistance RDSON and therefore being almost made like ideal switches.
The main advantages of the topology are, as it is well known, the very low voltage drop in conduction, typical of the bipolar transistors, and the turn-off speed. While turning off, in fact, the current output from the base is equal to the collector one, i.e., it is a very high current. This determines a drastic reduction both of the storage time and of the fall time, allowing the circuit to work also up to a frequency of 150 kHz.
Normally, the driving circuit also comprises an electrolytic capacitor and a zener diode parallel connected between a constant supply voltage reference and a resistance which connects such configuration in parallel with the control terminal of the bipolar transistor, as shown in FIG. 1.
The capacitor has the task of storing energy during the turn-off of the circuit for using it again during the subsequent turn-on and conduction of the power device; whereas the zener is a safety element for avoiding that the base voltage exceeds a determined value.
The driving operated by a circuit of this type is very useful and efficient in all those cases wherein the current on the device is void or very small with respect to the nominal one during the so-called turn-on.
FIG. 2 shows the wave forms which refer to a converter of the fly-back type which works at a frequency of 100 kHz and has a void turn-on current since the converter works in a discontinuous way.
When one is going to work with applications wherein the current value on the device at the turn-ON is not void, and at relatively high frequencies (>60 kHz), having to do with a bipolar transistor, the phenomenon of the VCESAT dynamics is highlighted. For this reason, with the driving circuit of FIG. 1, an excessive dissipation at the turn-on would be obtained due to the fact that the voltage drop VCE would take a relatively long time (>=2 μs) for reaching the saturation value.
With reference again to the circuit of FIG. 1, an increase of the VB would reduce only partially the problem of the VCESAT dynamics but it would worsen the turn-off performances to excess.
A suitable driving network for the applications with non void collector current at the turn-on has been subject of the European patent application no. 03425140.5, the disclosure of which is incorporated by reference, and is shown in FIG. 3.
The circuit structure of FIG. 3 suitably modulates the base current optimizing both switching steps and allowing the attainment of the lowest VCESAT value in the shortest possible time. FIG. 4 shows the wave forms which refer to a converter of the forward type which works at a frequency of 110 kHz, where the modulation of the base current can be observed.
The detailed operation and the sizing of the circuit of FIG. 3 are widely discussed in the cited patent application.
Although advantageous from several points of view, the driving circuit structured in the above schematically described way has known drawbacks mainly highlighted when the need of applications with collector currents having highly variable value is to be coped with.
In such applications, the circuit structure of the European patent application No. 03425140.5 must be sized taking into account the most stressful working condition, i.e., the highest collector current.
A correct saturation level should be thus ensured at the highest current value, by suitably choosing the Vb′ value, but, in so doing, for low collector current values, the device would work under over-saturation conditions, obtaining extremely long storage times.
This would imply an excessive dissipation at the turn-off, as well as a little accurate control due to the fact that the effective turn-off of the device would occur behind time with respect to the signal provided by the controller.
A need accordingly exists for devising a driving circuit of an emitter switching configuration to control the saturation level in applications which provide of highly variable collector currents, able to meet the above described need, i.e., that of having a base current proportional to the collector one so as to ensure a saturation level being suitable under each working condition, simultaneously overcoming, in a simple and effective way, all the drawbacks cited with reference to the prior art.